The invention relates to a device for generating an electron beam, which device has a main surface provided with an electrically insulating layer having at least one aperture within which the electron beam is generated and having a gate electrode provided along at least the greater part of the aperture in the electrically insulating layer.
The invention also relates to a support for such devices and to a cathode ray tube and a display device provided with such a device or support.
In addition to cathode ray tubes (display tubes, camera tubes), a device of the type described may also be adapted for electrolithographic applications or electron microscopy.
Netherlands Patent Application 7905470 laid open to public inspection and herein incorporated by reference shows a cathode ray tube provided with a semiconductor device, a so-called "cold cathode". The operation of this cold cathode is based on the emission of electrons from a semiconductor body in which a pn junction is reverse biased in such a way that there is an avalanche multiplication of charge carriers. Some electrons may then acquire so much kinetic energy as is required to exceed the electron work function. These electrons are then emitted at the main surface of the semiconductor body and thus provide an electron current.
The emission of the electrons in the device is simplified by providing the semiconductor device with so-called acceleration electrodes or gate electrodes on an insulating layer located on the main surface, which electrodes leave an aperture (slit-shaped, annular, round, rectangular) in the insulating layer. To facilitate the emission of the electrons the semiconductor surface is provided, if desired, with a material decreasing the work function such as, for example, cesium.
Such "cold cathodes" may be advantageously used in thin, flat display devices as described in Netherlands Patent Application 8700486 corresponding to U.S. Pat. No. 4,853,585, in which a number of electron beams is generated in a row of juxtaposed semiconductor cathodes. In these devices an associated row of electron beams is incident on a fluorescent screen after deflection, acceleration and further electron-optical operations and causes a row of pixels to luminesce in accordance with the information which has been presented. For a conventional pixel pitch of 750 .mu.m and, for example, a magnification of the emissive surface of approximately 30.times. in the electron-optical system, the positioning tolerance of the cathodes is therefore less than 10 .mu.m because otherwise the pixels may overlap one another (assuming that all emissive surfaces are located in one and the same plane). Such a tolerance imposes very strict requirements on the assembly.
In the device described in Netherlands Patent Application 8700486 corresponding to U.S. Pat. No. 4,853,585 the main surface of the cathodes extends substantially parallel to the surface in which the electron beams substantially move. Highly energetic positive ions can only partly reach the surface of the semiconductor cathodes, so that their efficiency is prevented from rapid deterioration due to the ion bombardment. This is achieved by deflecting the electron beam through 90.degree. by means of an electron-optical system comprising, inter alia an electron mirror.
The electron beam must be substantially parallel for a satisfactory operation of this electron mirror. Since the gate electrode usually functions as an acceleration electrode, it has a negative lens action on the beam of generated electrons. To render the beam substantially parallel, it is therefore necessary to arrange a first electrode preferably at the shortest possible distance from the cathode, which electrode has a positive lens action rendering the electron beam substantially parallel. The minimum distance at which such an electrode can be mounted (inter alia, due to the presence of bonding wires contacting the cathode) is approximately 300 .mu.m.
This causes great problems from an assembly-technical point of view. Moreover, due to this distance it is necessary to use such high voltages for the lens action and the mirror action of the first electrode and the mirror electrode, respectively, that positive ions are also generated between the mirror electrode and the cathode so that the efficiency of the cathode may be affected by an ion bombardment.